This invention generally relates to blow molding methods and machines for producing heat set plastic containers. More specifically, this invention relates to blow molding methods and machines for producing biaxially oriented plastic containers with high crystallinity sidewalls.
Recently, manufacturers of polyethylene terephthalate (PET) containers have begun to supply plastic containers for commodities that were previously packaged in glass containers. The manufacturers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities. Manufacturers currently supply PET containers for various liquid commodities, such as juices. They also desire to supply PET containers for solid commodities, such as pickles. Many solid commodities, however, require pasteurization or retort, which presents an enormous challenge for manufactures of PET containers.
Pasteurization and retort are both methods for sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above 70xc2x0 C., for duration of a specified length. Retort differs from pasteurization in that it also applies overpressure to the container. This overpressure is necessary because a hot water bath is often used and the overpressure keeps the water in liquid form above its boiling point temperature. These processes present technical challenges for manufactures of PET containers, since new pasteurizable and retortable PET containers for these food products will have to perform above and beyond the current capabilities of conventional heat set containers. Quite simply, the PET containers of the current techniques in the art cannot be produced in an economical manner such that they maintain their material integrity during the thermal processing of pasteurization and retort and during subsequent shipping.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related, in part, to the percentage of the PET container in crystalline form, also known as the xe2x80x9ccrystallinityxe2x80x9d of the PET container. Crystallinity is characterized as a volume fraction by the equation:   Crystallinity  =            ρ      -              ρ        a                            ρ        c            -              ρ        a            
where xcfx81 is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc).
The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse axis. The combination promotes biaxial orientation. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having roughly 20% crystallinity (average sidewall crystallinity).
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. Used by itself on amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable as the sidewall of the container). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a heated blow mold, (at a temperature of 120-130xc2x0 C.) and holding the blown container for about 3 seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85xc2x0 C., currently use heat setting to produce PET juice bottles having a range of up to 25-30% crystallinity. Although these hot fill PET containers exhibit a significant improvement over the non-hot fill PET containers, they cannot maintain material integrity during the thermal processing of pasteurization and retort.
A logical extension of this heat setting process involves blow molding a PET preform against a blow mold that is held at a considerably higher temperature, up to 250xc2x0 C., as discussed in the Jabarin references (U.S. Pat. No. 4,476,170 and U.S. Pat. No. 4,512,948). In theory, a manufacturer using this process could produce a PET container having over 50% crystallinity which allows the PET container to maintain its material integrity properties during a subsequent pasteurization or retort process of the contents in the PET container as well as during any subsequent shipment of the PET container. However, once this heat setting process has been completed, the PET container must be removed from the mold. At a temperature around 250xc2x0 C., upon removal of the PET container will instantly shrink and possibly collapse.
Recognizing this disadvantage, the Jabarin references offer two options for removing the PET containers: (1) lowering the mold temperature to the point where the PET container may be removed without any deformation, and (2) removing the PET container while applying internal pressure sufficient to resist any subsequent shrinkage thereafter and reducing the pressure when the bottle has reached a self-sustaining temperature. Neither of these options are commercially feasible. The first option involves extremely long cycle times (unless expensive liquid nitrogen machinery is employed) while the second option involves extremely complex processing to control the inherent variability of the system.
Thus, the manufacturers of PET containers desire an efficient and inexpensive method and apparatus that produces PET containers having high average sidewall crystallinities greater than 30%, which allow the PET containers to maintain their material integrity during any subsequent pasteurization or retort of the contents in the PET container, and during shipment of the PET containers. It is therefore an object of this invention to provide such a container that overcomes the problems and disadvantages of the conventional techniques in the art.
Accordingly, this invention provides for an efficient blow molding method and machine that produces PET containers having high average sidewall crystallinities of at least 30%, which allow the PET containers to maintain their material integrity during any subsequent high performance pasteurization or retort of the contents in the PET containers, and during shipment of the PET containers. As used herein, xe2x80x9chigh performancexe2x80x9d pasteurization and retort are pasteurization and retort processes where the container is exposed to temperatures greater than about 80xc2x0 C.
At its broadest, the invention is a method of producing a heat set plastic container including the steps of providing a plastic preform within a mold cavity; expanding and stretching the preform into conformity with surfaces defining the mold cavity; circulating a high-temperature gas through the interior of the plastic container to induce crystallinity in the plastic container; and mixing the high-temperature gas with a fluid such that the heat transfer coefficient of the high-temperature gas and the fluid mixture is greater than the heat transfer coefficient of the high-temperature gas.
The invention also includes a blow molding machine for producing a heat set container from a plastic preform according to the method mentioned above. Briefly, the machine includes a blow mold defining a mold cavity, which is capable of receiving a plastic preform. A high-temperature gas source and a fluid source communicate with a blow core assembly that engages the plastic preform. A mixer, which is coupled to the high-temperature gas source and to the fluid source, selectively mixes the high-temperature gas with the fluid and produces a mixture with a heat transfer coefficient that is greater than the heat transfer coefficient of the high-temperature gas. The blow core assembly further includes an exhaust to exhaust the mixture from the interior portion of the preform. A controller coupled to the high-temperature gas source and to the fluid source selectively controls the supply of the high-temperature gas and the fluid to the blow core assembly. The controller is also coupled to the exhaust to selectively control the exhaust of the mixture. By introducing a fluid into the high-temperature gas, the heat transfer coefficient of the high-temperature gas is effectively increased. Because of this increase, heat is transferred to the plastic container more rapidly and the temperature of the plastic container reaches a target temperature more quickly. Thus, by mixing a fluid into the high-temperature gas, the cycle time to produce a high crystallinity, heat set container may be reduced and efficiency may be increased.